on-mdi-activate in frame%
Called under Windows when a MDI-child frame becomes the active frame
within its parent (in which case the argument is #t), or when
the child frame ceases to be the active frame (in which case the
argument is #f).
MDI activation is different from keyboard-focus activation. If the
parent frame is the frontmost top-level frame, so that the MDI child
gets or loses the keyboard focus, then a separate
on-activate notification is sent to the MDI-child frame.